Structural Constraints, Spandrels, and Exaptation 1289
must be nearly neutral (or at least not too burdensome) because they "come with the
territory" and must immediately integrate themselves as parts of the larger structure
built at their level—for at-level spandrels share the same kind of form, and are made
of the same sorts of materials, as the primary adaptations themselves. If pendentives
made buildings collapse, or doubled their cost without compromising their
mechanical function, then architects would not choose the designs that generate them
as necessary consequences of otherwise favorable properties.
But cross-level spandrels, especially when injected upwards into more slowly
cycling biological individuals at higher levels, can establish themselves more easily,
and beyond any screening power of the higher-level individual, by pressure of
numerous introductions within single generations of the slowly-cycling higher-level
individual. I have been emphasizing the future exaptive potential of nonadaptive
spandrels, but nonadaptive features can also work in an opposite manner, becoming
detrimental (truly inadaptive) to their bearers—a fate that probably befalls at-level
spandrels only rarely (because inadaptive effects will generally preclude their
introduction in the first place), but that may represent a common outcome of cross-
level spandrels injected into higher-level individuals, and not readily suppressed, at
least initially, because they can become rooted before any episode of generational
cycling reveals their disutility to the higher-level individual.
The claims of the last paragraph may seem arcane and distant (in my abstract
formulation) from empirical reality. But this phenomenon has long been recognized
at the species level, even though evolutionary biology previously lacked the
conceptual apparatus to offer a general explanation. We all acknowledge that many
organismal adaptations impose strongly negative consequences upon the geological
longevity of their lineage. Any highly complex, metabolically expensive, and
intensely specialized adaptation (the peacock's tail, or virtually any elaborate
contrivance of runaway sexual selection); any alteration, especially involving the loss
and simplification of complex ancestral structures, that adapts an organism to a
transient and highly specialized environment (the "degenerate" parasite utterly
dependent on a unique and unusual host); must strongly compromise the geological
potential of a subclade bearing its autapomorphy, relative to a sister subclade
retaining an ancestral and generalized morphology and ecology. In fact, any feature—
and they must be legion—that provides adaptive benefits at the organismal level, but
that simultaneously injects such "negative" spandrels into the encompassing species-
individual (either suppressing its rate of speciation or decreasing its geological
longevity thereby), will be inadaptive at the higher level, but unpreventable by
insertion before the species-individual can "notice" and reject the feature.
This pairing of organismal adaptation with injected spandrels that prove
inadaptive to the encompassing species-individual sets the proper conceptual context,
under the hierarchical theory of selection, for what our literature has long called, in
an ambiguous and merely descriptive way, the "opportunism" of evolution.
"Opportunism," like "preadaptation," should be recognized as